This invention generally pertains to a connector for use in coupling a motor drive shaft to a driven shaft, and more specifically, in a medical drug infusion pump, to an elastomeric coupler for connecting a motor drive shaft to a cam shaft that drives a plunger in the pump, and to providing a restoring force for the plunger.
In many portable motor driven devices, small direct current (DC) motors are connected to rotatably driven shafts using solid metal couplers. Such couplers comprise a short section of thick-walled tubing having two threaded orifices in the tubing wall, adjacent each end. A set screw is threaded into each orifice. The set screws are tightened to engage a drive shaft of the motor that is inserted into one end of the coupler, and to secure a driven shaft that is inserted into the other end of the coupler. Even if a fastener locking substance is applied, the set screws often loosen with use, causing scoring of the shafts and possible failure of the devices in which the couplers are installed.
Couplers are generally available from suppliers in only a limited range of sizes. If the coupler used to join two shafts is too large, it will not properly connect the shafts and can cause vibration, because its mass is not symmetrically distributed around the center lines of the two shafts. In addition, conventional couplers generally require that the center lines of the two shafts that are joined be relatively closely aligned. Thus, for example, any misalignment between a motor drive shaft and a driven shaft, even if slight, is likely to cause side loading of one or both shafts, producing greater bearing wear. Solid couplers also transmit noise and vibration from the motor to other parts of the device in which they are used.
Ideally, it would be preferable to provide a more flexible coupling between a motor drive shaft and a driven shaft. Large motor couplers sometimes include fiber reinforced rubber assemblies clamped around the ends of two shafts to provide some degree of flexibility, but such couplers are too large for use in small devices.
It will therefore be apparent that a simple coupler, which addresses the problems noted above and is relatively low in cost, would be desirable for use in small electric motor powered devices. The prior art does not provide a suitable alternative to the prior art solid metal connector of the type described, or the too large and cumbersome prior art fiber reinforced rubber connector assemblies.
In accord with the present invention, a coupler is defined for connecting a non-cylindrical end of a drive shaft to a non-cylindrical end of a driven shaft. The coupler includes a generally cylindrical fitting having opposed first and second ends. A first opening is disposed at the first end of the fitting, and a second opening is disposed at the second end of the fitting. The first opening has a size and a shape generally corresponding to a size and a shape of the non-cylindrical end of the drive shaft. Likewise, the second opening has a size and a shape generally corresponding to a size and a shape of the non-cylindrical end of the driven shaft. The fitting is formed of an elastomeric material adapted to elastically stretch when the first opening is forced over the drive shaft and when the second opening is forced over the driven shaft, providing an interference fit in each case. The fitting is thereby adapted to drivingly couple the drive shaft to the driven shaft.
Preferably, the fitting further comprises a web that is disposed transverse to a longitudinal axis of the fitting, between the first opening and the second opening. This web limits a depth to which the drive shaft and the driven shaft are advanced into the first opening and the second opening, respectively. Also, the web limits vibration of the drive shaft propagating into the driven shaft from the drive shaft.
A sleeve that is sized to snugly fit around an outer surface of the fitting in an interference fit provides a compression force that helps to keep the fitting seated on the drive shaft. An outer surface of the sleeve includes a cam surface adapted to act on a plunger. As the drive shaft rotates the driven shaft, the sleeve causes the plunger to move along a longitudinal axis of the plunger. Preferably, the cam surface is adapted to only apply a force against the plunger in one direction; the plunger is biased in an opposite direction by an elastomeric membrane which the plunger displaces.
An outer surface of the fitting is keyed to an inner surface of the sleeve. Also, an inner surface of the first opening includes a flat section adapted to seat on a corresponding flat section formed on the end of the drive shaft. In a similar manner, an inner surface of the second opening includes a flat section adapted to seat on a corresponding flat section formed on the end of the driven shaft.
The elastomeric fitting thus minimizes vibration transmission between the drive shaft and the driven shaft and helps to minimize audible noise. Because of its elasticity, the fitting can accommodate at least a limited degree of misalignment between the drive shaft and the driven shaft.